1. Field of the Invention
The present invention relates to an optically isotropic pitch suitable as a raw material for carbon fibers or activated carbon fibers and to a process for producing the same.
More specifically, the invention relates to an optically isotropic pitch suitable as a raw material for carbon fibers or activated carbon fibers which has good spinning ability and has no fear of fusion of fibers during the course of the infusibilization stage, and to a process for producing said optically isotropic pitch by subjecting a pitch (including a heavy oil) to a two-stage heat-treatment.
Furthermore, the invention is to provide an optically isotropic pitch having substantially no anisotropic component and having a low QI (quinoline insoluble) content as the raw material for carbon fibers or activated carbon fibers.
2. Related Art
An optically isotropic pitch as a raw material for carbon fibers requires to contain no primary quinoline insolubles (primary QI components), which are said to hinder the spinning due to their infusibility and insolubility, and to have a high softening point in order to carry out the infusibilization treatment after the spinning smoothly.
Consequently, up to now, a pitch having a high softening point is obtained by removing primary QI components, etc. from a pitch or a heavy oil by means of filtration, etc., and then distilling it or heat-treating it under the conditions where primary QI components and optically anisotropic components are not formed.
However, the softening point of the heat-treated pitch obtained by these methods has not yet been sufficient for smoothly carrying out the spinning of fibers.
Yet, there is a technique where a heavy oil or pitch having the primary QI components removed is heat-treated while blowing therein a gas containing oxygen, etc. (hereinafter referred to as "air-blowing"), whereby a pitch having a relatively high softening point and possessing good properties in terms of spinning pitch (Japanese Patent Laid-Open Sho 61-28020).
In spite of possessing a high softening point, however, it is difficult for the pitch obtained in this manner to suppress the formation of optical anisotropic components, and because of the high content of low boiling point component, there is a likelihood of some kind of trouble in the course of the stage for spinning carbon fibers, the fusion in the course of infusibilization stage, and the low carbonization yield after the sintering stage.
However, it is difficult to be converted into a high softening point in any pitch obtained by the prior art's process without forming any optically anisotropic component. Accordingly, since the pitch has a low softening point and contains large amounts of light fractions having a low boiling point, the fusion of the fibers is brought about in the course of infusibilization at a high heat-up rate.
Moreover, because of large amounts of light components, the pitch possesses the disadvantage of poor spinning characteristics due to the stain of the nozzle, etc.
In order to improve this, a technique where a heavy oil or a pitch having the primary QI components removed is heat-treated while conducting the air-blowing under a reduced pressure to obtain an optically isotropic pitch (Japanese Patent Laid-Open Sho 63-156886), and a technique where an optically isotropic pitch is obtained in a similar process by using a high viscosity reactor capable of being continuously maintained under a reduced pressure (Japanese Patent Laid-Open Sho 63-156887) have been developed, but both techniques have a problem in that the load of the treatment under a reduced pressure is high, the yield of the resulting optically isotropic pitch is low, the cross-linked bonding of the pitch by air-blowing does not proceed, and optically anisotropic components tend to be formed.